Heterochromatin regulation is critical for genomic stability. Different H3K9 methylation states have been discovered, with distinct roles in heterochromatin formation and silencing. However, how the transition from H3K9me2 to H3K9me3 is controlled is still unclear. Here, we investigate the role of the conserved bromodomain AAA-ATPase, Abo1, involved in maintaining global nucleosome organisation in fission yeast. We identified several key factors involved in heterochromatin silencing that interact genetically with Abo1: histone deacetylase Clr3, H3K9 methyltransferase Clr4, and HP1 homolog Swi6. Cells lacking Abo1 cultivated at 30 °C exhibit an imbalance of H3K9me2 and H3K9me3 in heterochromatin. In abo1∆ cells, the centromeric constitutive heterochromatin has increased H3K9me2 but decreased H3K9me3 levels compared to wild-type. In contrast, facultative heterochromatin regions exhibit reduced H3K9me2 and H3K9me3 levels in abo1∆. Genome-wide analysis showed that abo1∆ cells have silencing defects in both the centromeres and subtelomeres, but not in a subset of heterochromatin islands in our condition. Thus, our work uncovers a role of Abo1 in stabilising directly or indirectly Clr4 recruitment to allow the H3K9me2 to H3K9me3 transition in heterochromatin.

异染色质调节对于基因组稳定性至关重要。已发现不同的H3K9甲基化状态，在异染色质形成和沉默中具有不同的作用。但是，如何控制从H3K9me2到H3K9me3的过渡仍然不清楚。在这里，我们调查保守的bromodomain AAA-ATPase，Abo1，参与维持裂变酵母中的全球核小体组织的作用。我们确定了与Abo1基因相互作用的异染色质沉默的几个关键因素：组蛋白脱乙酰基酶Clr3，H3K9甲基转移酶Clr4和HP1同源Swi6。缺少在30°C下培养的Abo1的细胞在异染色质中显示H3K9me2和H3K9me3的不平衡。在abo1中Δ细胞，着丝粒组成型异染色质与野生型相比，H3K9me2含量增加，但H3K9me3含量降低。相反，兼性异染色质区域在abo1Δ中表现出降低的H3K9me2和H3K9me3水平。全基因组分析表明，在我们的条件下，abo1 ∆细胞在着丝粒和亚端粒中均具有沉默缺陷，但在异染色质岛的子集中则没有。因此，我们的工作揭示了Abo1在直接或间接稳定Clr4募集以允许异染色质中从H3K9me2过渡到H3K9me3的作用。